A purpose of a brush in an electrical device is to pass electrical current from a stationary contact to a moving contact surface, or vice versa. Brushes and brush holders may be used in electrical devices such as electrical generators, electrical motors, and/or slip ring assemblies, for example, slip ring assemblies on a rotating machine such as a rotating crane. Brushes in many electrical devices are blocks or other structures made of conductive material, such as graphite, carbon graphite, electrographite, metal graphite, or the like, that are adapted for contact with a conductive surface or surfaces to pass electrical current.
In some designs, a brush box type brush holder is used to support the brush during operation. The brush and box are designed such that the brush can slide within the box to provide for continuing contact between the brush and the conductive surface contacted by the brush. As the brush slides within the box, one or more outer surfaces of the brush may rub against the inner surface of the brush box, which can create deposits of brush material on the inner surface of the brush box. Furthermore, during wear of a brush, fine particles and/or dust can be created, which can collect on nearby surfaces and the inside of the brush box, between the inner surface of the brush box and the brush, and also can create deposits of brush material on the inside of the brush box. Such deposits can restrict sliding movement of the brush within the box, which in turn can reduce the quality of the contact between the brush and the contact surface. A build-up of deposits may adversely affect the wear rate of the brush and/or the conductive surface contacting the brush.
Additionally, some brush designs include one or more electrical shunts or wires to provide an electrical current path from the brush to another structure. In at least some designs, the one or more electrical shunts are typically attached to the brush opposite the wear surface by a tamping or riveting method. Over time, the brush will be reduced in size, or get shorter, for example as the wear surface of the brush in frictional contact with the conductive surface wears down. As the brush is worn, the distance between the wear surface of the brush contacting the moving conductive surface and the attachment point of the shunt is reduced. A brush creates a certain amount of electrical resistance, which is dependent on the distance between the wear surface and the attachment point of the shunt. For example, a new brush may have an initial length that creates the greatest resistance through the brush because the distance between the wear surface and the attachment point of the shunt is greatest in a new brush. As the brush wears, the distance between the wear surface and the attachment point of the shunt is reduced, thus reducing the resistance through the electrical pathway extending through the brush. This variability in resistance can be undesirable.
A number of different brushes and brush holder structures, assemblies, and methods are known, each having certain advantages and disadvantages. However, there is an ongoing need to provide alternatives.